Monday, June 20, 2011

What is a Stack?

The term Stack has many meanings, flavors, … ask 10 experts and you may get 11 definitions. People talk about an IEC 61850 stack, an OPC UA stack, .... What do these mean? Are they comparable?

Let’s start with the general definition:

According to the Wikipedia: “The protocol stack is an implementation of a computer networking protocol suite. The terms are often used interchangeably. Strictly speaking, the suite is the definition of the protocols, and the stack is the software implementation of them.”

So, the software that processes the protocols is called the (protocol) stack.

With regards to IEC 61850 this can mean many things: Session, Presentation, ACSE, MMS, ACSI, MMS-SCSM, Model management and configuration language, API to the application, … let’s have a look at the server side of the communication:

“Protocol” aspects

Remarks and Explanations

1

API to application

Control of Server SW, local services for read, write, events, control, … “Protocol” that defines how the application can communicate with the underlying IEC 61850 software.

2

Models, model management and model and ACSI configuration language

Describe the server’s information model and binding to application (LDs, LNs, DO, DA, …) Be aware that LNs have also services and protocols (see below for LN GLOG). The information models has to be organized in the IED’s software (including retrieving the self-description of the model)

The ACSI services use MMS to carry the payload between client and server. MMS provides the serialization of (service) messages. Example: A Buffered Report Control Block is a quite comprehensive “Service” model with a set of service parameters (for control block attributes). The state machine of the Control Block requires a bit of a software!

What is the Logical Node GLOG? An application or a model with services and protocol (messages)? The GLOG is a standardized application that defines a model, services and a protocol! Guess you did not expect this … others may not agree with me …

IEC 61850-7-4 Edition 2 defines:

“5.7.4 LN: Generic log Name: GLOG The LN GLOG refers to a function which allows to log not only changed data itself but also any related data being defined in the settings of LN GLOG. The logging is started by the changed data object (TrgRef1) or by the operator (LogTrg). The logged data are identified by the references to the related source data objects in the data model.” This in short the state machine of the GLOG service model and protocol (in abstract terms). The GLOG communicates with a client via services and a protocol …

The logged Data Values will be stored in an IEC 61850 Log … it can be queried by services from a client.

Let’s come back to our question, what is implemented in a stack?

Stacks from different vendors may be for free, may be reasonable priced, or may be expensive! What does this mean? Almost nothing! Because the CRUCIAL question is: WHAT would you get for your Euros or Dollars?

A stack of vendor X may cover the implementation described under bullets 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10.

A stack of vendor Y may cover only 5, 6, 7, 8, 9, and 10.

The difference is tremendous: The efforts to implement the requirements listed in bullets 1, 2, 3, and 4 are (to my experience) likely more than 90 … 95 per cent of what needs to be implemented with regard to IEC 61850!

If you hear something like “the stack so-and-so is cheaper …” listen twice and then think about what you have heartthree times and ask what that stack really provides four timesAND ASK PEOPLE WITH EXPERIENCES WHAT IS LEFT FOR YOU TO DO to get a compliant IED !!! I have talked to many experts that were surprised that it took sooo long … and cost sooooo … much to get a compliant IED.

When it comes to the comparison of OPC UA and IEC 61850: Listen very carefully, and ask questions … and then … and then you may understand the difference from a standard and from an implementation point of view.

Click HERE if you want to experience what could be provided by a specific stack providing integrated software for issues 1 to 10 … with little left for you [German]. A workshop in English may be set up when you are interested … let Beck IPC know that you would attend a workshop in English.